c++ - Implementing other comparison operators in terms of operator< in one call

Question

I've seen that, if you have operator<, you can implement, say, operator>= as !(a < b). However, when I went to the C++ Committee in Bristol, they said that you can implement all the comparison operators this way. Particularly, when considering types which have non-trivial comparison operators (consider an arbitrarily long string, especially if this is Unicode), this can be done with only one call to operator<. I cannot conceive of how this could be done.

How can I implement operator> and operator<= with just one call to operator< and none to the other comparison operators?

Answer 1

• a > b == b < a
• a <= b == !(b < a)
• a >= b == !(a < b)

It's even possible to implement equality in terms of less than (Kind of abusing my meta-syntax here):

• (a == b) == (!(a < b) && !(b < a))
• (a != b) == (a < b || b < a)

Although I wouldn't suggest doing so in practice, since it requires two comparisons and can generally be implemented more efficiently directly.